The regulation of the heartbeat is commonly viewed in terms of the opposing influences of the autonomic nervous system neurotransmitters, acetylcholine and noradrenalin. Acetylcholine decreases the rate and force of heartbeat whereas noradrenalin has the opposite effects. In addition to the well-studied effects of the autonomic neurotransmitters, a variety of other chemical substances have pronounced effects on cardiac excitability. In this proposal, we plan to investigate one class of these substances: the adenine compounds. Adenine compounds have potent inhibitory effects on the heart which are very similar to those of acetylcholine. Recently, I have shown that micromolar concentrations of adenosine produce slow hyperpolarizations in the muscle cells in the primary pacemaker region of the frog heart. This hyperpolarization is produced by interaction of adenosine with specific receptors in the surface membrane of cardiac muscle cells. Despite the clear evidence for the existence of these receptors in the heart, virtually nothing is known about the physiological function of adenosine in regulation of the heartbeat or the molecular mechanisms by which adenosine produces its effects. This proposal aims to investigate these questions. Specifically, electrophysiological experiments are designed to elucidate (a) the ionic mechanisms of the response of heart muscle to adenosine and (b) the possible physiological roles adenosine may play in the heart (is adenosine a neurotransmitter?). Biochemical experiments will investigate the role of cyclic nucleotides and membrane phosphorylation in mediating the slow hyperpolarization produced by adenosine.